1. Field of the Invention
The present invention relates to a method of and apparatus for controlling current of an inverter, and more particularly, to a method of and apparatus for controlling current of the inverter in which output currents of the inverter connected to a three-phase load are detected, and control is effected so that instantaneous values of the output currents are substantially equal to set output current command values, respectively.
2. Description of the Prior Art
FIG. 1 shows a conventional apparatus for controlling a current-controlled three-phase inverter which feeds back output currents of the inverter to effect current control so that instantaneous values of the output currents are substantially equal to output current command values, respectively. A three-phase inverter 1 includes switching elements 1a, 1b, 1c connected to a DC power source in parallel. Output terminals a, b, c of the switching elements are connected through wires to the respective phases of a Y-connected three-phase load 2. Further, each of the wires is connected with a current detector 3 for detecting instantaneous values of output currents i.sub.a, i.sub.b, i.sub.c delivered from the switching elements, respectively. The current detector 3 is connected to a current detection circuit 4 for changing output signals from the current detector 3 into signal levels suitable for use in current control circuits 6, 7, 8, respectively. A calculating circuit 5, which calculates output current command values i.sub.aR, i.sub.bR, i.sub.cR for the respective output currents of the switching elements, is connected to the current control circuits 6, 7, 8 for ON/OFF controlling the switching elements independently of one another. The current control circuits 6, 7, 8 are fed with the output currents i.sub.a, i.sub.b, i.sub.c from the current detection circuit 4, and are connected to the switching elements, respectively.
As shown in FIG. 2, each of the current control circuits 6, 7, 8 is constituted by a series circuit composed of a comparator 9 for making a comparison between an output current command value and the instantaneous value of an output current, an amplifier 10 with hysteresis and a driver 11.
In the inverter current controlling apparatus arranged as above, the switching elements are ON/OFF controlled by feedback control so that the output currents of the switching elements are substantially equal to the output current command values, respectively. Since the switching elements are switched to the anode or cathode of the DC power source through the ON/OFF control, there are eight combinations in polarity of the output terminals a, b, c of the inverter as shown in Table 1 below.
TABLE 1 ______________________________________ Voltage vector Output terminal .fwdarw.V.sub.0 .fwdarw.V.sub.1 .fwdarw.V.sub.2 .fwdarw.V.sub.3 .fwdarw.V.sub.4 .fwdarw.V.sub.5 .fwdarw.V.sub.6 .fwdarw.V.sub.7 ______________________________________ a - + + - - - + + b - - + + + - - + c - - - - + + + + ______________________________________
Accordingly, the phase voltage or phase potential applied to the load 2 is constituted by eight kinds of voltage vectors V.sub.0 to V.sub.7, shown in Table 1 and FIG. 3, which are made temporally continuous.
FIG. 4A shows waveforms of the output current command values i.sub.aR, i.sub.bR, i.sub.cR and those of ideal phase voltage v.sub.a, v.sub.b, v.sub.c applied to the load when currents completely coincident with the respective current command values are delivered from the inverter. The waveforms of the ideal phase voltages are out-of-phase with respect to those of the respective output current command values due to the impedance of the load. Further, FIGS. 4A to 4D are enlarged views of the part A of FIG. 4A which is divided into three so as to be shown in these Figures, respectively. FIG. 4B shows the waveforms of a potential E.sub.a at the output terminal a of the switching element 1a, the output current i.sub.a and the output current command value i.sub.aR, while FIG. 4C shows the waveforms of a potential E.sub.b at the output terminal b of the switching element 1b the output current i.sub.b and the current command value i.sub.bR. In addition, FIG. 4D shows the waveforms of a potential E.sub.c at the output terminal c of the switching element 1c, the output current i.sub.c and the current command value i.sub.cR. Referring to FIG. 4B, when the output current i.sub.a exceeds a upper-limit UTP in the case of rising of the current, the switching element 1a is turned OFF and connected to the cathode of the DC power source; when the output current i.sub.a is less than a lower-limit LTP in the case of decaying of the current, the switching element 1a is turned ON and connected to the anode of the DC power source. This ON/OFF control causes the potential E.sub.a at the output terminal a to change in the form of pulse. On the other hand, when the switching element 1a is ON or OFF, the other switching elements are turned ON or OFF; hence, the output current i.sub.a stepwisely changes as shown in FIG. 4B. In addition, the voltage vectors applied to the load when the switching elements are thus controlled, are constituted by voltage vectors in six directions, that is, V.sub.4, V.sub.5, V.sub.6, V.sub.1, V.sub.2, V.sub.3 which are made temporally continuous.
As described above, in the conventional inverter current controlling apparatus, the potentials at the output terminals a, b, c of the switching elements are selected independently of one another. Therefore, the voltage vectors in six directions are freely selected with respect to all of periods. In consequence, the output current waveform includes an exceedingly large amount of ripple, resulting in the production of strident noises in the operation of the inverter as well as an increase of electro-magnetic wave, which causes a noise to an electronic circuit, disadvantageously. Further, since the number of required ON/OFF operations of the inverter, that is, the number of commutating operations thereof required for effecting current control is increased, the switching elements consituted by semiconductors or the like increase in switching loss, resulting in a reduction in conversion efficiency of the inverter and an increase in the inverter capacity, disadvantageously. Moreover, when the load is an electric motor, since the voltage vectors in six directions are selected with respect to all of periods as described above, there is a period during which voltage vectors in the direction opposite to an ideal magnetic flux rotation direction are undesirably selected. In consequence, the magnetic flux continuously repeats normal rotation, reverse rotation and stop, so that the locus of the magnetic flux unfavorably draws a wastefully curved loop and includes many vibrations. As a result, the iron loss is increased, and the electric motor is further reduced in efficiency, disadvantageously. In addition, when the load is an electric motor, there is a problem of increase in torque pulsation and copper loss due to the current ripple, in addition to the above-mentioned problems.